The present invention relates to a variable gain alternating voltage amplifier having a d.c. feedback loop provided for setting the operating point of the amplifier.
Controllable amplifiers are required, for example, to suppress noise in audio frequency signals in compander systems.
A simple circuit with good control properties for a controllable amplifier is an amplifier with a controllable feedback loop. In order to set the operating point of such an amplifier, it may be necessary to provide a d.c. feedback loop. If, in this case, the desired change in gain of the amplifier were provided by variation of the resistance of a controllable resistor disposed in the d.c. feedback loop this would have the result, for example, that relatively fast changes in gain would appear as simulated signals at the input of the amplifier.
The reason for this is that in the above-described case a change in the negative feedback resistance would have the same effect as a change in the operating point. This again would have the same effect as a correspondingly changing signal at the input of the amplifier.